Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015		Sep. 30, 2014		Dec. 31, 2014	[1]
Segment Reporting Information [Line Items]
Total Assets		$ 12,627,962		$ 12,627,962				$ 16,723,738
External Revenues		714,949	$ 1,433,037	2,374,986		$ 4,068,458
Income (Loss)		(1,595,426)	245,707	(1,683,701)		530,373
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		12,575,600		12,575,600
External Revenues		714,900	1,433,000	2,375,000		4,068,400
Income (Loss)		(1,587,100)	271,000	(1,672,500)		583,000
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		52,400		52,400
Income (Loss)		(8,300)	(25,300)	(11,200)		(52,600)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets		1,416,800		1,416,800
External Revenues		49,300	2,100	94,000		8,600
Income (Loss)		9,000	(40,400)	(44,000)		(139,800)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	11,158,800		11,158,800
External Revenues	[2]	665,600	1,430,900	2,281,000		4,059,800
Income (Loss)	[2]	(1,596,100)	311,400	(1,628,500)		722,800
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	5,935,200		5,935,200
External Revenues	[2]	335,100	667,600	955,000		1,660,400
Income (Loss)	[2]	(107,800)	130,500	(218,100)		335,300
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	2,540,500		2,540,500
External Revenues	[2]	123,200	246,900	428,400		807,400
Income (Loss)	[2]	(507,000)	40,400	(577,800)		160,900
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets		2,527,800		2,527,800
External Revenues		207,300	516,400	897,600	[2]	1,592,200	[2]
Income (Loss)		(952,700)	148,000	(701,900)	[2]	482,600	[2]
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	155,300		155,300
External Revenues	[2]					(200)
Income (Loss)	[2]	$ (28,600)	$ (7,500)	$ (130,700)		$ (256,000)

[1] Reclassified to conform to current presentation.

[2] Additional details about results of oil and gas operations are presented in the table on pages 25 and 26.